Highly Tetravalent Hafnium Ion Conducting Solids with a NASICON-Type Structure

Abstract

Tetravalent Hf⁴⁺ ion conducting solid electrolytes, Hf_1−x/4(Nb_1−yW_y)_5/(5+y)P_3−xW_xO₁₂, were developed by partially replacing both the Nb⁵⁺ and P⁵⁺ sites in the NASICON-type HfNb(PO₄)₃ solid simultaneously with higher valence W⁶⁺ ions. Among the samples prepared, the highest conductivity was obtained for the Hf_3.85/4(Nb_0.8W_0.2)_5/5.2P_2.85W_0.15O₁₂ (y = 0.2; x = 0.15) solid, which contains a high amount of W⁶⁺ ions and holds the optimum lattice volume for Hf⁴⁺ ion conduction, enabling a significant reduction of the electrostatic interaction between the Hf⁴⁺ cation and the O²⁻ anions in the solid. The conducting species in the Hf_3.85/4(Nb_0.8W_0.2)_5/5.2P_2.85W_0.15O₁₂ solid was directly demonstrated to be the Hf⁴⁺ ion via DC electrolysis.